Tomorrow morning, in a building on the edge of the City of London close to Liverpool Street station, the atmosphere will be extremely edgy. The 'congestion charging hub' will be 'going live' from 7am.

Mayor Ken Livingstone may be the man with the most to lose. But Rod Aldridge, executive chairman of the Capita Group, which will run the scheme, comes a close second.

This FTSE 100 company is used to controversy and regularly handles the flak for government computing failures such as the Criminal Records Bureau fiasco. Its share price has halved during a difficult past 12 months. Analysts wondered whether Aldridge had taken a gamble too far in accepting the politically fraught challenge of the congestion charge.

Last week's attempts by local newspapers to infiltrate the Coventry call centre which takes payments for the charge was just an indication of public relations catastrophes to come. If the scheme works then Capita will be the mayor's enforcers; if there's a failure in the system, it will surely shoulder all the blame.

'When you are introducing something on this scale, there will always be a few teething problems,' says Jonathan Hawker of Capita. 'We have been going through numerous end-to-end dummy runs, it has been tested to extraordinary levels.'

And the London-based firm stands to be the pioneer in a market worth up to £6 billion.

Transport for London set the parameters of Capita's contract in considerable detail. It determined that stationary cameras and not satellite tracking (as used in Singapore) would be the means of recognition. TfL also chose the cameras from Initial Electronic Security Systems - the same as used by the City of London for its 'ring of steel'.

Capita runs the back office system. Each camera in the zone is connected by fibre optic cable to the congestion charging hub. There are two types - infra-red cameras to identify the number plate and colour cameras to take images of the car for evidential purposes. Stills of the cars are taken from the rolling video footage provided by the cameras. Each one has its own computer server at the hub.

The automated numberplate recognition (ANPR) software will recognise and interpret four plates per second, even if they are strategically covered by mud. The software then bundles each photo with a time and loca tion stamp and feeds it to a central database.

From 7pm that 'plate database' is continually cross-referenced against a separate database of drivers who have paid the charge, others who are exempt, and those with resident discounts. Call centres in Coventry and Glasgow take payments as well as a website, a text message service, postal payment and Paypoint's charging system at local retailers.

There is automatic cross- matching between the two databases, gradually eliminating all the vehicles that have been paid for. The numberplates of the remaining cars are sent to the DVLA in Swansea, which returns information of the drivers' names, addresses and car types. These data are then sent back to Coventry. Capita employees manually check the images, and send out first penalty notices for £80 if there is a match.

'None of the individual technologies used are completely brand new, but the key element is the way in which we put this together,' says Hawker. 'We own all the intellectual property on this solution.'

The patents are crucial to Capita's plans. Although this contract is worth £230 million over five years, creating jobs for about 1,300 people, there are bigger prizes. Ken Living stone has hinted that the scheme could be rolled out across a larger chunk of London. And there are 30 British cities keenly awaiting the result of the experiment.

'Our scheme is fully scaleable and adaptable,' says Capita's spokesman. 'The start-up costs are far less than a Singapore-style satellite tracking system. We see this as more than just a contract, a golden opportunity to gain ground in the UK as a whole.'

When Capita announces last year's preliminary results on Thursday - thought to be a respectable £100m in pre-tax profits - it will reveal the extent of its ambitions. It is likely to create a consultancy company to advise the world's great international cities on its congestion charging system.

But it is still excited about possibilities within Britain. Capita believes that its solution is also applicable as an alternative to motorway and road tolls. Much will depend on tomorrow's launch.